Carbon surface modification for three-volt ultracapacitor

ABSTRACT

An electric double-layer ultracapacitor configured to maintain desired operation at an operating voltage of three volts, where the capacitor includes a housing component, a first and a second current collector, a positive and a negative electrode electrically coupled to one of the first and second current collectors, and a separator positioned between the positive and the negative electrode. At least one of the positive electrode and the negative electrode can include a treated carbon material, where the treated carbon material includes a reduction in a number of hydrogen-containing functional groups, nitrogen-containing functional groups and/or oxygen-containing functional groups.

CROSS REFERENCE TO RELATED APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57. Thisapplication is a continuation of U.S. application Ser. No. 15/619,123,filed Jun. 9, 2017, which is a divisional of U.S. application Ser. No.14/047,818, filed Oct. 7, 2013, which claims the benefit of priorityunder 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No.61/711,072, filed Oct. 8, 2012, U.S. Provisional Patent Application Ser.No. 61/711,046, filed Oct. 8, 2012, U.S. Provisional Patent ApplicationSer. No. 61/710,943, filed Oct. 8, 2012, and U.S. Provisional PatentApplication Ser. No. 61/711,142, filed Oct. 8, 2012, each of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION Field

The present invention relates generally to electrical energy storagedevices, and, more specifically, to the design of electrodes forelectrical energy storage devices, such as double-layer capacitors.

Description of the Related Art

Electrical energy storage cells are widely used to provide power toelectronic, electromechanical, electrochemical, and other usefuldevices. Such cells include primary chemical cells, secondary(rechargeable) cells, fuel cells, and various species of capacitors,including ultracapacitors. Some characteristics of electrical energystorage cells include energy density, power density, charging rate,internal leakage current, equivalent series resistance (ESR), andability to withstand multiple charge-discharge cycles. For a number ofreasons, capacitors that can store relatively large amounts of charge,known as supercapacitors and ultracapacitors, have gained prominenceamong the various electrical energy storage cells.

Increases in operating voltage of ultracapacitors can provide enhancedenergy storage and power capabilities. However, various components ofthe capacitor may exhibit instabilities when subject to operatingconditions of increased voltage. For example, instabilities in one ormore components of the capacitor may contribute to deterioration incapacitor performance, including but not limited to excessivecapacitance fade and Equivalent Series Resistance (ESR) increases oncycling or in DC lifetime also referred to as calendar lifetime,self-discharge, pseudocapacity, and/or gas formation.

SUMMARY

One embodiment is a capacitor configured to operate at a voltage of 3volts or greater and including: a first current collector and a secondcurrent collector, a positive electrode electrically coupled to thefirst current collector, and a negative electrode electrically coupledto the second current collector, at least one of the positive electrodeand the negative electrode including treated carbon material. Thetreated carbon material can include a reduction in a number of one ormore functional groups on one or more surfaces of the treated carbon,for example about 10% to about 60% reduction in a number of one or morefunctional groups. The capacitor may also include a separator positionedbetween the positive electrode and the negative electrode, anelectrolyte in ionic contact with the positive electrode and thenegative electrode, and a housing component to retain the positiveelectrode, the negative electrode, the separator and the electrolyte. Insome embodiments, the capacitor can be configured to provide a desiredperformance while operating the capacitor at 65° C. with a cycle life ofgreater than 500 k cycles. In some embodiments, the capacitor can beconfigured to provide a desired performance while operating at 3 voltsor greater at 65° C. and maintaining greater than 80% of its initialcapacitance and/or less than 200% of its initial equivalent seriesresistance for over 1500 hours.

The treated carbon can include a reduced number of hydrogen-containingfunctional groups, nitrogen-containing functional groups, and/oroxygen-containing functional groups. In some embodiments, the treatedcarbon material comprises functional groups less than about 1% of whichcontains hydrogen, including less than about 0.5%. In some embodiments,the treated carbon material comprises functional groups less than about0.5% of which contains nitrogen, including less than about 0.1%. In someembodiments, the treated carbon material comprises functional groupsless than about 5% of which contains oxygen, including less than about3%.

In some embodiments, the treated carbon material can include about a 30%reduction in a number of hydrogen containing functional groups.

Another embodiment is a method of making an ultracapacitor configured tooperate between about 2.8 volts to about 3 volts. The method includestreating a carbon material to result in a treated carbon material havingabout 10% to about 60% reduction in a number of at least one functionalgroup on a surface of the treated carbon material; forming a firstelectrode and a second electrode from the treated carbon material;separating the first electrode and second electrode with a separator;and inserting the first electrode and second electrode into a housing toform an ultracapacitor. In some embodiments, the treated carbon materialhas about 20% to about 50% reduction in a number of one or morefunctional groups.

One other embodiment is an ultracapacitor having a first currentcollector and a second current collector; a positive electrodeelectrically coupled to the first current collector; a negativeelectrode electrically coupled to the second current collector, where atleast one of the positive electrode and the negative electrode includesa treated carbon material having a number of functional groups on asurface of the treated carbon material less than about 1% of whichcontains hydrogen, less than 0.5% of which contains nitrogen, or lessthan about 5% of which contains oxygen; a separator positioned betweenthe positive electrode and the negative electrode; an electrolyte inionic contact with the positive electrode and the negative electrode;and a housing component to retain the positive electrode, the negativeelectrode, the separator and the electrolyte and wherein theultracapacitor is capable of operating at 3 volts at a temperature of65° C. or greater while maintaining less than 200% of its initialequivalent series resistance for over 1500 hours.

For purposes of summarizing the invention and the advantages achievedover the prior art, certain objects and advantages are described herein.Of course, it is to be understood that not necessarily all such objectsor advantages need to be achieved in accordance with any particularembodiment. Thus, for example, those skilled in the art will recognizethat the invention may be embodied or carried out in a manner that canachieve or optimize one advantage or a group of advantages withoutnecessarily achieving other objects or advantages.

All of these embodiments are intended to be within the scope of theinvention herein disclosed. These and other embodiments will becomereadily apparent to those skilled in the art from the following detaileddescription having reference to the attached figures, the invention notbeing limited to any particular disclosed embodiment(s).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure are described with reference to the drawings of certainembodiments, which are intended to illustrate certain embodiments andnot to limit the invention.

FIG. 1 is a block diagram showing a simplified cross-sectional view ofan example electric double-layer ultracapacitor.

FIG. 2 is a cross-sectional perspective view showing an example housingcomponent of an electric double-layer ultracapacitor.

FIG. 3A shows capacitance performance of an electric double-layerultracapacitor having a barrier film coating a portion of a housingcomponent interior surface.

FIG. 3B shows a resistance performance of an electric double-layerultracapacitor having a barrier film coating a portion of a housingcomponent interior surface.

FIG. 4 shows a capacitance performance of an electric double-layerultracapacitor having a reduced electrolyte concentration.

FIG. 5 shows a cross sectional top view of electric double-layerultracapacitor in a jelly-roll configuration.

FIG. 6 is a perspective view showing various layers of an electricdouble-layer ultracapacitor in a jelly-roll configuration.

FIG. 7 is a line graph showing the results of modifying the carbonsurface characteristics of an ultracapacitor electrode.

FIG. 8 is a cross-sectional view showing a simplified example electricdouble-layer ultracapacitor comprising asymmetric electrodes.

DETAILED DESCRIPTION

Although certain embodiments and examples are described below, those ofskill in the art will appreciate that the invention extends beyond thespecifically disclosed embodiments and/or uses and obvious modificationsand equivalents thereof. Thus, it is intended that the scope of theinvention herein disclosed should not be limited by any particularembodiments described below.

FIG. 1 shows a simplified cross-sectional view of a portion of anexample electric double-layer ultracapacitor 10. The example portion ofdouble-layer ultracapacitor 10 includes a first electrode 22, forexample a positive electrode, having a first active electrode portion14, and a second electrode 24, for example a negative electrode, havinga second active electrode portion 18. A separator 16 is positionedbetween the electrodes 22, 24 to maintain a separation between the firstelectrode 22 and the second electrode 24. An electrode may comprise acurrent collector to facilitate electrical contact between the electrodeand an external circuit. Referring to FIG. 1, for example, the positiveelectrode 22 includes a first current collector 12 electrically coupledto the first active electrode portion 14 and a second current collector20 is electrically coupled to the second active electrode portion 18.The separator 16 and both electrodes 22, 24 may be immersed in anelectrolyte (not shown). The electrolyte may permeate the separator 16and the active electrode portions 14, 18, facilitating for example ionmobility between the electrodes 22, 24.

Embodiments of the invention relate to technologies for increasing theoperating voltage of an ultracapacitor to 3 volts or greater. Thesetechnologies, as described below, allow an ultracapacitor to operate at3 volts or greater, while minimizing the disadvantageous side effectsthat such a relatively high voltage can have on the ultracapacitor. Inparticular, increasing voltage has been found to cause an increase inthe rate of secondary electrochemical reactions that occur within theultracapacitor. For example, secondary electrochemical reactions canoccur between an ultracapacitor electrolyte (e.g., an acetonitrile basedelectrolyte) and one or more other materials of the ultracapacitor.These reactions have been found to be sensitive to operating voltages,and the reaction rate can increase as voltage on the capacitor isincreased. The byproducts of the reactions can cause increased build-upof gas of various constituents within the ultracapacitor, including forexample H₂ and CO₂, which may cause pressure build-up within the deviceand/or leaking of the device. In addition, in ultracapacitors havingaluminum current collectors, the aluminum current collector maycontribute to the secondary reactions, which can result in an increasedinternal resistance of the device. Finally, the secondary reactions mayresult in physical and/or chemical modification of the carbon-basedelectrodes and electrode structures, which has a detrimental impact onthe ultracapacitor performance, including for example, a capacitance ofthe ultracapacitor.

The following technologies, used separately, or in combination with oneanother can alleviate or reduce the issues caused by operating anultracapacitor at 3 volts or greater, particularly at highertemperatures, such as 60° C., 65° C., 70° C., 75° C. or greatertemperatures while maintaining more than 80% of its initial capacitanceover 1500 hours, more than 80% of its initial capacitance over a numberof cycles (e.g., cycling between a rated voltage of the capacitor to ahalf voltage), less than 200% of its initial ultracapacitor equivalentseries resistance (ESR) over 1500 hours, and/or less than 200% of itsinitial ultracapacitor equivalent series resistance (ESR) over a numberof cycles (e.g., cycling from a rated voltage of the capacitor to a halfvoltage). In some embodiments, the technologies described herein, usedseparately or in combination with one another, facilitates operation ofan ultracapacitor at 3 volts while demonstrating less than a targetleakage current over a period of time (e.g., less than a leakage currentof 18 milliamperes (mA) over a number of hours, such as over 72 hours)and/or demonstrating a self discharge of less than 25% over a period oftime, such as over 72 hours.

Technologies described herein may also be used separately or incombination with one another to enable a desired operation of anultracapacitor at an operating voltage of 3 volts or greater at atemperature of about 65° C. for greater than or equal to 500 k cycles.For example, an ultracapacitor may include one or more technologiesdescribed herein to enable the ultracapacitor to maintain orsubstantially maintain a capacitance greater than about 80% of itsinitial capacitance, and/or less than 200% of its initial equivalentseries resistance when operating at a voltage of 3 volts or greater overa period of about 1,500 hours, and/or over greater than or equal to 500k cycles, and at a temperature of about 65° C. In other embodiments, theultracapacitor is enabled to maintain at least 75%, 85%, 90%, 95% or 99%of its initial capacitance when operating for a period of 1500 hours,and/or over greater than or equal to 500 k cycles at 65° C. or greater.

Definitions

As used herein, capacitance (F—Farads) is a measurement of energystorage in joules. C=qV

As used herein, the voltage is the maximum operating voltage for asingle capacitor. The rated voltage is the voltage at which theperformance data is measured. It is possible for the capacitors toexperience voltages in excess of the rated voltage. The impact isdependent on the time and temperature during this exposure.

As used herein, Surge Voltage is the maximum voltage that anultracapacitor can operate at for a short periods of time with minimaldamage or cell opening.

As used herein, Internal Resistance (DC) is the resistance correspondingto all the resistive components within the ultracapacitor, R_(tot). Thismeasurement is taken at the end of the discharge of the device andspecifically some number of seconds, typically 5 seconds after thedischarge current has ceased to flow (ESR=ΔV/abs(I)). Since the timeconstant of the ultracapacitors is approximately 1 second, it takesapproximately 5 time constants or 5 seconds to effectively remove 99.7%of the stored energy. R_(tot) is comprised of resistive componentsattributed to contact or interconnect resistance, electrode conductiveresistance, electrolyte conductive and ionic resistance, and othermaterial resistances.

As used herein Cycle Life is the expected performance characterizationof an ultracapacitor when cycled from rated voltage to half ratedvoltage for 50 k cycles, 500 k cycles, 1 million (M) cycles or anynumber of cycles when cycling the ultracapacitor between rated voltageto half rated voltage. In one embodiment, cycling is performed at a dutycycle and current level which results in no or substantially no rise inthe internal and/or external body temperature of the ultracapacitor withthe ultracapacitor maintaining a temperature equal to or substantiallyequal to 65° C.

Housing with Interior Coating

Referring to FIG. 2, an electric double-layer ultracapacitor 29 mayinclude a housing component 30, configured to receive and retain apositive electrode, a negative electrode, a separator and electrolyte asdiscussed above in reference to FIG. 1. The housing component 30 mayinclude one or more walls 32, a base 36 and an upper lid 38 having anexterior surface 40 and an interior surface 42, where the interiorsurface 42 defines at least in part a housing interior space 34configured to retain the positive electrode, the negative electrode, theseparator and the electrolyte. For example, the housing component 30 maycomprise a cylindrical container having a side wall 32, a base 36 and anupper lid 38. The side wall 32, base 36 and/or the upper lid 38 may bemade of a conductive material. For example, the base 36 and/or the upperlid may include conductive material that allows for electrical contactwith one or more current collectors (e.g., the current collectors 12 and20, as shown in FIG. 1) to enable current flow from the currentcollectors to an external circuit. The conductive material of the sidewall 32, the base 36 and/or the upper lid 38 may comprise aluminum,nickel, silver, steel, tantalum, other suitable metallic material,and/or combinations thereof. The housing component 30 may also take theform of other shapes (e.g., a housing component having a prismaticshape) and it not just limited to cylindrical shapes.

The interior surface 42 of the side wall 32 may be in contact withelectrolyte. This may result in the chemical and/or electrochemicalinteraction between the electrolyte and the interior surface 42, forexample generating byproducts which may degrade a performance of thecapacitor. In one embodiment of the invention, a barrier film 44 may beapplied to at least a portion of the interior surface 42 of the housingcomponent 30 that is exposed to the electrolyte to provide a protectivecoating for the one or more interior surfaces 42.

The barrier film 44 may be applied to a portion of a side wall 32interior surface, an interior surface of base 36, and/or an interiorsurface of upper lid 38. In some embodiments, the barrier film 44 may beapplied to all or substantially all interior surface portions of thehousing component 30 that are exposed to the electrolyte. In someembodiments, a portion of the base 36 and/or the upper lid 38 isuncoated or substantially uncoated by the barrier film 44, for exampleto reduce a degradation of electrical coupling between active componentsof the ultracapacitor and an external circuit.

In some embodiments, the barrier film 44 is applied to all orsubstantially all of the interior surface 42 of the side wall 32, exceptportions of the interior surface 42 of the side wall 32 configured toprovide contact between active components of the ultracapacitor and anexternal circuit. For example, all or substantially all of the interiorsurface 42 of the side wall 32 may be coated by the barrier film 44except portions of the interior surface 42 of the side wall 32configured to provide electrical coupling to one or more currentcollectors of the ultracapacitor. In some embodiments, the uncoatedportion of the interior surface 42 of side wall 32 can have an areaconfigured to provide reduced interference by the barrier film 44 uponelectrical coupling between the active components of the ultracapacitorand the external circuit (e.g., an area configured to reduce adegradation of the electrical coupling between the one or more currentcollectors of the ultracapacitor and the external circuit), whileproviding desired coating of the interior surface 42 of the side wall 32for facilitating reduced interaction with the electrolyte.

For example, a portion (e.g., a portion forming a band along the base 36having a width) of the interior surface 42 of the side wall 32 proximateto and/or along the base 36 of the housing 30 may be uncoated by thebarrier film 44. In some embodiments, the uncoated band has a width ofless than about 10 millimeters (mm). For example, an uncoated portion ofthe interior surface 42 of the side wall 32 along the base 36 of thehousing 30 can form a band having a width of about 5 mm in width. Ofcourse other shapes and/or dimensions of uncoated portions may also besuitable. In some embodiments, the uncoated portion of the interiorsurface 42 of the side wall 32 can be at another location on the sidewall 32.

In some embodiments, the barrier film 44 enables reduced chemical and/orelectrochemical interaction between the one or more interior surfaces 42and the electrolyte (e.g., facilitating a reduction in corrosion of theone or more interior surfaces 42). For example, the barrier film 44 mayexhibit chemical resistance to the electrolyte and may provide aphysical barrier between an interior surface 42 and the electrolyte soas to enable a reduction in the interaction between the interior surface42 and the electrolyte, and a reduction in the generation of byproducts.In some embodiments, the barrier film 44 may prevent or substantiallyprevent all chemical interactions between the interior surface 42 andthe electrolyte, particularly if the interior surface 42 is an aluminumsurface.

The barrier film 44 may comprise a material having a desired mechanicalstrength, sufficient adhesion to the underlying interior surface 42 ofthe housing component 30, and/or chemical and/or electrochemicalstability with respect to the electrolyte. In some embodiments, thebarrier film 44 is free or substantially free of pin-holes, cracks,and/or other defects.

The barrier film 44 can have a sufficient thickness to provide desiredseparation between the electrolyte and the interior surface 42 whileproviding a coating having a reduced impact on a volume of the housinginterior space 34 occupied by the barrier film 44. A volume of thehousing interior space 34 occupied by the barrier film 44 may reduce avolume available for other components of the ultracapacitor. In someembodiments, the barrier film 44 is uniformly or substantially uniformlyapplied to portions of the interior surface 42. A variety of thicknessesmay be suitable. For example, the barrier film of about 5 micrometers(μm) to about 40 μm. In some embodiments, the barrier film 44 can have athickness of about 5 μm to about 55 μm. For example, the barrier film 44applied to a portion of the interior surface 44 of the sidewall 32 canhave a thickness of about 10 μm.

In some embodiments, the barrier film 44 can include an electricallynon-conductive material, such as a polymeric material. For example, thebarrier film 44 may comprise polyepoxide (e.g., epoxy resin), polyolefin(e.g., polypropylene (PP), polyethylene (PE, e.g., high-densitypolyethylene (HDPE), low-density polyethylene (LDPE)), polyimide,polyether ether ketone (PEEK), polyurethane, ethylene propylene rubber(EPDM, EPR), poly(p-xylylene) (e.g., parylene), fluorinated polymers(e.g., polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF),ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene(FEP)), copolymers thereof, and/or combinations thereof.

In some embodiments, the barrier film 44 can include an electricallyconductive film. The electrically conductive film may enable the portionof the interior surface 42 to maintain or substantially maintain adesired electrical contact with another ultracapacitor component (e.g.,a current collector) and/or an external circuit. In some embodiments,the film includes an electrically conductive material having chemicaland/or electrochemical resistance to the electrolyte, for example aconductive carbon material. The electrically conductive film can be madeof graphite (e.g., graphitic ink, graphitic slurry, graphene). Forexample, the film can include a mixture comprising an electricallyconductive carbon material (e.g., graphite and/or graphene) and a bindermaterial (e.g., a thermoplastic binder material). In some embodiments,the mixture can be a dispersion of the electrically conductive carbon ina thermoplastic binder, such as a water-based dispersion. For example,the barrier film 44 can be made of a water-based dispersion of graphitein a thermoplastic binder material. The barrier film 44 made of aconductive carbon can have a desired electrical conductivity, forexample a sheet resistance of less than about 30 ohms per square(“Ω/sq”) at a thickness of about 25 μm.

In some embodiments, the barrier film 44 can include an electricallynon-conductive film, an electrically conductive film, and/or acombination thereof. For example, the barrier film 44 can include onlyan electrically non-conductive material (e.g., one or more polymericfilms), only an electrically conductive material (e.g., one or moreelectrically conductive films made of a conductive carbon material), oran electrically non-conductive film over an electrically conductive film(e.g., one or more polymeric films over one or more electricallyconductive carbon-based films). For example, a portion of interiorsurface 42 may be coated with a barrier film 44 including anelectrically conductive film beneath a polymeric film, the electricallyconductive film facilitating a reduction in interference by thepolymeric film with an electrical contact between the portion of theinterior surface 42 and a current collector of the ultracapacitor.

In some embodiments, an aluminum interior surface can be anodized toform a protective oxide barrier layer. In some embodiments, a stainlesssteel interior surface can be passivated to provide a barrier layer.

FIG. 3A shows capacitance performances (e.g., capacitance fadeperformance) of ultracapacitors having portions of an interior surfaceof their housing coated by a barrier film. The barrier file was anelectrically conductive barrier film made of a carbon material,including graphite. The ultracapacitors had portions of an interiorsurface of a housing sidewall coated by the barrier film to reduce theinteraction between the sidewall and the electrolyte while also reducinginterference with electrical coupling between active components of thecapacitor and an external circuit. The comparison is shown between theultracapacitors having the barrier coating and control ultracapacitorshaving no barrier film on any portion of their housing interiorsurfaces. FIG. 3A shows that ultracapacitors that were coated with agraphite conductive barrier film had improved capacitance over time. Forexample, ultracapacitors having an interior surface of their sidewallcoated by a barrier film demonstrated were found to retain about 75% oftheir capacitance at 1000 hours of operation at 3 volts and 65° C. incomparison to uncoated ultracapacitors which only retained about 68% oftheir capacitance under the same conditions at 1000 hours of operation.The coated ultracapacitors were found to be able to operate until about1500 hours before reaching the same approximate 68% capacitance showingthat the coating extended the capacitance lifetime for ultracapacitorsrunning at 3 volts and 65° C.

FIG. 3B shows the electrostatic resistance performance (e.g., ESR) ofultracapacitors having the interior surface of their housing coated by abarrier film made of carbon graphite in comparison to ultracapacitorshaving no barrier film. FIG. 3B shows that coated ultracapacitors wereable to maintain a lowered ESR over time in comparison to uncoatedultracapacitors. A shown, the ESR of the coated ultracapacitors at 1500hours of operation at 3 volts and 65° C. was lower than the ESR ofuncoated ultracapacitors at only 1000 hours of operation under the sameconditions.

Suitable methods to apply the barrier film 44 to an interior surfaceportion of the housing component 30 can include, for example, dipcoating, spray coating, brush coating, liquid dispersion coating, vapordeposition, spin coating, wiping on, painting on and/or dripping.

Electrolyte

As described herein, electric double-layer ultracapacitors includeelectrolyte that is capable of transporting ions between a positiveelectrode (e.g., the positive electrode 22 shown in FIG. 1) and anegative electrode (e.g. the positive electrode 24 shown in FIG. 1). Theelectrolyte may be a solution having a solvent and a salt, with the saltproviding ionic species for ionic conductivity and contact between thepositive electrode and the negative electrode. A suitable electrolytemay also exhibit a low viscosity and/or a high degree of ionicconductivity, thereby enabling a decreased capacitor internal resistanceand increased capacitor voltage during charging and discharging of thecapacitor. For example, an increased solubility of the salt in thesolvent may enable increased ionic conductivity between the positive thenegative electrodes. A suitable electrolyte may exhibit chemical and/orelectrochemical stability under the operating conditions of theultracapacitor and may be able to withstand repeated charge dischargecycles of the ultracapacitor.

Electrolyte with an Acetonitrile Solvent and Various Salts

One embodiment of the invention is an ultracapacitor that exhibitsstable performance when operating at 3 volts or more and includes anelectrolyte that has a salt which exhibits increased solubility in theelectrolyte solvent and can be chemically and/or electrochemicallystable at the 3 volt operating voltage. This electrolyte may enableincreased ion mobility, and/or may exhibit increased chemical and/orelectrochemical stability under operating conditions of theultracapacitor. In one example, the capacitor electrolyte solvent isacetonitrile.

In some embodiments, an electrolyte salt can include an ionic liquid.For example, a suitable electrolyte salt can include an ionic liquidwhich demonstrates desired stability at an operating voltage of 3 voltsor higher, provides sufficient ionic conductivity between electrodes ofthe ultracapacitor, and/or demonstrates desired solubility in anacetonitrile electrolyte solvent.

An electrolytic salt may include a quaternary ammonium salt havingdesired solubility in an acetonitrile solvent.

In some embodiments, a cation of the electrolytic salt comprisesspiro-(1,1′)-bipyrrolidinium (SPB), triethylmethyl ammonium (TEMA),and/or tetraethyl ammonium (TEA). In some embodiments, an anion of theelectrolytic salt comprises tetrafluoroborate and/or iodide. Forexample, the electrolyte may include a salt comprisingspiro-(1,1′)-bipyrrolidinium tetrafluoroborate, triethylmethyl ammoniumtetrafluoroborate, tetraethyl ammonium tetrafluoroborate,spiro-(1,1′)-bipyrrolidinium iodide, triethylmethyl ammonium iodide,and/or tetraethyl ammonium iodide. For example, an electrolytecomprising triethylmethyl ammonium tetrafluoroborate, tetraethylammonium tetrafluoroborate, and/or spiro-(1,1′)-bipyrrolidiniumtetrafluoroborate can provide an electrolyte having increased saltsolubility and improved chemically and/or electrochemically stability atincreased operating voltages, where the electrolyte comprises anacetonitrile solvent.

In some embodiments, a cation of the electrolyte salt can include asymmetrical cation, including for example, triethylmethyl ammonium. Insome embodiments, a cation of the electrolyte salt can include anasymmetrical cation, including for example, triethylmethyl ammonium. Insome embodiments, the electrolyte salt can include spiro compounds,including symmetrical and asymmetrical spiro compounds. For example, theelectrolyte can include N-spirobicyclic compounds, including symmetricalN-spirobicyclic compounds having 5-member cyclic rings. In someembodiments, the electrolyte can include an asymmetrical spiro compound,including asymmetrical spiro compounds having unequal sized ringstructures. A symmetrical spiro compound can include aspiro-(1,1′)-bipyrrolidinium tetrafluoroborate.

Electrolyte with Reduced Salt Concentration

Another embodiment of the invention is an ultracapacitor that canprovide desired operation at an operating voltage of 3 volts or greaterwherein the ultracapacitor includes an electrolyte having a lowered saltconcentration in comparison to typical ultracapacitors. The electrolytemay include a salt having improved solubility in the electrolytesolvent, for example improved solubility in an acetonitrile-basedsolvent, such that the electrolyte may have a reduced salt concentrationwhile maintaining or substantially maintaining a desired ionicconductivity between a positive electrode and a negative electrode ofthe ultracapacitor. This allows a reduction in the concentration of theions while providing sufficient ionic transport between the electrodeswithout starving the ultracapacitor. An electrolyte having a reducedsalt concentration may enable a reduced chemical interaction between theelectrolyte and one or more other components of the ultracapacitor. Thereduced chemical interaction may, for example, enable a reduced a rateof byproduct generation and therefore provide an improved ultracapacitorperformance.

In some embodiments, the electrolyte with a reduced salt concentrationenables the electrolyte to have a reduced electrolyte concentrationnon-homogeneity. The non-homogeneity can be created under non-uniformcurrent densities within the ultracapacitor. The electrolyteconcentration non-homogeneity may facilitate salt precipitation onto oneor more electrode surfaces. This precipitation can, for example occludeone or more electroactive sites on an electrode and negatively impactthe performance of the ultracapacitor. By using an electrolyte with areduced salt concentration, the ultracapacitor may avoid an electrolyteconcentration non-homogeneity when the ultracapacitor is operating atincreased voltages, for example under high current charge and dischargecycling.

The electrolyte may include spiro-(1,1′)-bipyrrolidiniumtetrafluoroborate, tetraethyl ammonium tetrafluoroborate (TEA),triethylmethyl ammonium tetrafluoroborate (TEMA),spiro-(1,1′)-bipyrrolidinium iodide, triethylmethyl ammonium iodideand/or tetraethyl ammonium iodide, in a solvent comprising acetonitrile.In this embodiment, the electrolyte may have a concentration within arange of about 0.5 moles/L (M) to about 1.0M, including from about 0.7 Mto about 0.9M. For example, the electrolyte may include a 0.8M solutionof tetraethyl ammonium tetrafluoroborate, triethylmethyl ammoniumtetrafluorborate and/or spiro-(1,1′)-bipyrrolidinium tetrafluoroboratein acetonitrile.

FIG. 4 is a line graph showing a comparison of capacitance performancebetween ultracapacitors having different electrolyte concentrations. Asshown, several ultracapacitors having an electrolyte solution of 0.8Mtetraethyl ammonium tetrafluoroborate in acetonitrile (TEA) werecompared to several ultracapacitors having 1.0 M TEA to measurecapacitance fade over time. Each ultracapacitor was run at 3.0 volts and65° C. As shown, ultracapacitors having a reduced electrolyteconcentration of 0.8M were found to have a reduced capacitance fadeovertime in comparison to ultracapacitors having a 1M TEA electrolyte.As shown at 500 hours, the ultracapacitors with 0.8M TEA hadapproximately a 78% capacitance fade whereas the ultracapacitors with1.0M TEA were found to have an about 84% capacitance face.

Other suitable solvents may comprise gamma-butyrolactone,dimethoxyethane, N,N,-dimethylformamide, hexamethyl-phosphorotriamide,propylene carbonate, dimethyl carbonate, tetrahydrofuran,2-methyltetra-hydrofuran, dimethyl sulfoxide, dimethyl sulfite,sulfolane (tetra-methylenesulfone), nitromethane, and/or dioxolane.Other suitable salts may comprise methyltriethylammoniumtetrafluoroborate, tetrabutylammonium tetrafluoroborate,tetraethylammonium hexafluorophosphate tetraalkylphosphonium salts(e.g., tetraethylphosphonium tetrafluoroborate, tetrapropylphosphoniumtetrafluoroborate, tetrabutylphosphonium tetrafluoroborate,tetrahexylphosphonium tetrafluoroborate, tetraethylphosphoniumhexafluorophosphate tetraethylphosphonium trifluoromethylsulfonate)and/or lithium salts (e.g., lithium tetrafluoroborate, lithiumhexafluorophosphate, lithium trifluoromethylsulfonate).

Separator

As described herein, an electric double-layer capacitor may have aseparator immersed in an electrolyte and positioned between the positiveelectrode and the negative electrode (e.g., the positive and negativeelectrodes, 22, 24, shown in FIG. 1). The separator enables electricalisolation of one electrode from another in an ultracapacitor, forexample preventing an electrical short between the positive electrodeand the negative electrode, while allowing transport of ions between thetwo electrodes. For example, the separator may comprise a porousmaterial with sufficient wettability to the electrolyte so as tofacilitate ion mobility between the positive and negative electrodes.The separator may be made of a material having mechanical strength,chemical stability and/or electrochemical stability, including forexample, a material which maintains or substantially maintains itsphysical, electrical and/or chemical properties under conditions of themanufacturing process and/or the operating conditions of the capacitor.In some embodiments, the separator is made from cellulose, including forexample cellulose fiber.

A separator can have a thickness optimized to provide a sufficientseparation between electrodes of the ultracapacitor while having areduced volume (e.g., for facilitating an ultracapacitor with a reducedvolume and/or weight, and/or an ultracapacitor having increased volumefor its other components). For example, a separator can have a thicknessof about 20 μm to about 50 μm. For example, a separator made ofcellulose can have a thickness of about 30 μm.

Electrode

A capacitance of the electric double-layer ultracapacitor depends atleast partially on the surface area available to store electricalcharge, and particularly the surface area available on the electrodes ofthe capacitor. Referring to FIG. 5, an electric double layerultracapacitor 50 is illustrated having a separator 54 positionedbetween a positive electrode 52 and a negative electrode 56. In thiscase, the ultracapacitor 50 is shown to have been wound or rolled into a“jelly-roll” configuration. The jelly-roll configuration may enablestorage of an increased surface area in a compact space. In someembodiments, the positive electrode 52 and/or the negative electrode 56comprise active portions of the ultracapacitor comprising porousmaterial having very large effective area per unit volume, i.e., verylarge normalized effective surface area.

Optimized Electrode Carbon Microporosity, Mesoporosity and Macroporosity

In another embodiment, the ultracapacitor running at 3 volts may haveone or more carbon based layers (e.g., carbon based layers 82, 84 ofFIG. 6) in an ultracapacitor electrode that includes a carbon materialhaving a desired microporosity (e.g., pores having a diameter of lessthan about 2 nanometers (nm)), mesoporosity (e.g., pores having adiameter of about 2 nm to about 50 nm) and/or macroporosity (e.g., poreshaving a diameter of greater than about 50 nm). For example, theelectrode may be made at least partially of activated carbon having anoptimized microporosity, mesoporosity and/or macroporosity for improvedperformance at three volts or more. The microporosity, mesoporosityand/or macroporosity of the carbon based layers may be optimized tofacilitate ion mobility within the layers and/or a capacitance value ofthe layers. Ions within an electrolyte of the capacitor may migratewithin electrodes of the capacitor after a number of charge-dischargecycles, becoming trapped within pores (e.g., micropores) of anelectrode. Trapped ions may become unavailable for further chargedischarge cycles and may contribute at least in part to a deteriorationin an operating characteristic of the capacitor, including for examplean increase in an equivalent series resistance (ESR) value and/or areduction in the capacitance performance. In some embodiments, thecarbon material can have a reduced microporosity to reduce a space inwhich ions may be trapped. Reducing a microporosity of a carbon basedlayer may facilitate access of ions to active surfaces of the electrode.A carbon based layer can have an optimized microporosity, mesoporosity,and/or macroporosity to reduce trapping of ions within pores of thecarbon based layers, while providing desired carbon based layercapacitance performance.

In some embodiments, a carbon based layer can have a microporosity ofabout 60% to about 85%, for example about 70% or about 75% or about 80%.In some embodiments, a carbon based layer can have a mesoporosity ofabout 10% to about 35%, for example about 20% or about 25%. In someembodiments, a carbon based layer can have a macroporosity of less thanabout 5%, for example about 1%. In some embodiments, a reduction inmicroporosity and an increase in a mesoporosity can provide improved ESRperformance and/or capacitance performance. For example, a reduction ofmicroporosity by about 20% to about 25% may facilitate a reduction in anESR value by about 20% to about 30% for an ultracapacitor operating at 3volts or higher and at about 65° C. In some embodiments, a carbon basedlayer of an electrode can have a microporosity within a range of fromabout 70% to about 85%, a mesoporosity of about 10% to about 30%, and amacroporosity within a range of less than about 5%, for example about1%.

In some embodiments, a compacted carbon based layer having an optimizedcomposition, packing density, microporosity, and/or macroporosity isadhered to one or more surfaces of a current collector (e.g., thecurrent collectors 84, 82 shown in FIG. 6). For example, a first and/orthe second carbon based layers may be adhered onto a surface of thecurrent collector through a laminating process. The first and/or secondcarbon based layers may be adhered directly or indirectly onto a surfaceof the current collector, for example, optionally including an adhesivelayer (e.g., a conductive adhesive layer) to promote adhesion between asurface of the current collector and the carbon based layer, and/or toenable a reduced sheet resistance. For example, the adhesive layercomprises a solvent, an adhesive component (e.g., a thermoplasticmaterial), and/or an electrical conductivity enhancer (e.g., graphiteand/or other conductive carbon powder). The adhesive layer may also beobtained commercially. For example, a compacted first carbon based layerhaving an optimized composition, packing density, microporosity, and/ormacroporosity may be adhered to a first surface of the current collectorand a compacted second carbon based layer having an optimizedcomposition, packing density, microporosity, and/or macroporosity may beadhered to a second surface of the current collector opposite to thefirst surface. In some embodiments, the carbon based layers have anoptimized composition, packing density, microporosity, and/ormacroporosity enables improved equivalent series resistance, carbonbased layer's structural integrity, improved ion mobility and/orincreased ultracapacitor cycle lifetime.

Asymmetric Electrode Thickness

In one embodiment of the invention, an ultracapacitor includes apositive electrode having a thickness greater than a thickness of anegative electrode. As shown in FIG. 8, an electric double-layercapacitor 150 may comprise a positive electrode 152 having a firstthickness T1 and a negative electrode 154 having a second thickness T2.In some embodiments, the first thickness T1 of a positive electrode 152is greater than a second thickness T2 of a negative electrode 154.

For example, a first sub-capacitor may be formed at an interface betweenthe positive electrode 152 and the electrolyte (e.g. the electrolyte 26shown in FIG. 1), and a second sub-capacitor may be formed at aninterface between the negative electrode 154 and the electrolyte. Anexample method to select the first and second thicknesses T1, T2comprises determining a positive voltage limit for the firstsub-capacitor and a negative voltage limit for the second sub-capacitor,dividing the positive voltage limit by the negative voltage limit toobtain a first ratio of the second sub-capacitor to the firstsub-capacitor, and setting relative thicknesses of the positiveelectrode layer and the negative electrode layer so that capacitance ofthe second sub-capacitor is substantially equal to a product of thefirst ratio and the capacitance of the first sub-capacitor. Furtherembodiments regarding one or more methods of determining the first andsecond thicknesses T1, T2 are provided in U.S. Patent ApplicationPublication No. 2006/0148112, which is incorporated herein by referencein its entirety.

In some embodiments, a carbon based layer (e.g., a carbon based layercan be made of a activated carbon, a binder material and/or anconductive additive) of the positive electrode 152 can have a thicknessgreater than a thickness of a carbon based layer of the negativeelectrode 154 by about 20 μm to about 100 μm. For example, a thicknessof a carbon based layer of the positive electrode 152 can be about 80 μmto about 200 μm, and a thickness of a carbon based layer of the negativeelectrode 154 can be about 60 μm to about 160 μm. For example, a firstthickness T1 of the positive electrode 152 can include a thickness of acurrent collector of the positive electrode 152 and a thickness of acarbon based layer on each of the two opposing surfaces of the currentcollector. A second thickness T2 of the negative electrode 154 caninclude a thickness of a current collector of the negative electrode 154and a thickness of a carbon based layer on each of the two opposingsurfaces of the current collector of the negative electrode 154.

In some embodiments, the first thickness T1 of a positive electrode 152is greater than the second thickness T2 of a negative electrode 154 by apercentage within a range of from about 15% to about 40%, including fromabout 20% to about 30%. For example, the first thickness T1 is greaterthan the second thickness T2 by about 25%. For example, the firstthickness T1 is greater than the second thickness T2 by about 35%.

In some embodiments, an ultracapacitor having asymmetric electrodes(e.g., a positive electrode and a negative electrode having differentthicknesses) can demonstrate improved capacitance and/or resistanceperformance. For example, an ultracapacitor having a positive electrodethicker than a negative electrode can demonstrate an improvement in acapacitance performance of about 5% to about 10% and/or an improvementin a resistance performance of 5% to about 10%, when operated at 3 voltsor greater, and at a temperature of about 65° C.

Carbon Surface Modification

Reduction of Carbon Surface Functional Groups

In one embodiment of the invention, carbon used in one or moreelectrodes of an ultracapacitor is treated to reduce the number ofcarbon surface functional groups to enable an improved ultracapacitorperformance. As described herein, in some embodiments, an electrode(e.g. the positive and/or negative electrodes 152, 154 shown in FIG. 8)comprises activated carbon. The high surface areas on the activatedcarbon may be created through a process of chemical and/or thermaloxidative processes. The incomplete oxidation of the carbon may createoxygen-containing functional groups on the carbon surface includingcarboxyl, carboxylate, hydroxyl, lactone, quinone and phenols. Theresidual oxygen may contribute to deleterious performance properties ofa capacitor. For example, residual oxygen may contribute to capacitancefade on cycling, self-discharge, pseudocapacity, gas formation at highpotential voltages and/or increased hydrophilic surface properties whichstimulate moisture adsorption. In some embodiments, surface functionalgroups having nitrogen (N) and/or hydrogen (H) may degrade performanceof the ultracapacitor during its operation.

In some embodiments, treated carbon material used in an electrode (e.g.the positive and/or negative electrodes 152, 154 shown in FIG. 8)comprises a reduced number of surface functional groups. For example,activated carbon material used in an electrode may be exposed to areactant gas (e.g., a reactant gas comprising CO₂ and/or N₂) andmicrowave energy at a temperature of at least 300° C. to reduce thenumber of functional groups residing on the treated carbon material. Insome embodiments, a reactant gas comprises a reducing gas. In someembodiments, an electrode carbon material may be treated at a hightemperature with a reactant gas comprising a reducing gas to enable, forexample, a reduced number of carbon surface functional groups. Forexample, the electrode carbon material may be treated at a temperaturewithin a range of from about 300° C. to about 1000° C., including fromabout 500° C. to about 1000° C. For example, activated carbon materialof the electrode may be treated at a temperature within a range of fromabout 500° C. to about 1000° C. with a reducing gas to enable areduction in a number of activated carbon surface functional groups.Further description regarding methods of carbon surface modification isprovided in U.S. Patent Application Publication No. 2009/0097188, whichis incorporated herein by reference in its entirety.

In some embodiments, a treated carbon material can have functionalgroups less than about 1% of which contains hydrogen (e.g., less thanabout 0.5%), less than about 0.5% of which contains nitrogen (e.g., lessthan about 0.1%), and/or less than about 5% of which contains oxygen(e.g., less than about 3%). In some embodiments, a treated carbonmaterial can have about 20% to about 50% reduction in the number offunctional groups on one or more surfaces of the treated activatedcarbon. For example, a treated carbon material can have about 30% fewerhydrogen-containing functional groups.

In one example, ultracapacitors with varying functional groups weretested for the amount of pressure each ultracapacitor built up over timewithin the device. The following table shows the carbon functionalmodifications for each test carbon. Each pressure test was performed at3 volts and 65° C.

Carbon [O] [N] [H] Carbon 1 0.68 0.41 0.55 Carbon 2 0.74 0.49 0.69Carbon 3 0.90 0.57 0.88

As shown in FIG. 7, improved performance was found for electrodes havingcarbons with lower surface functional group concentrations. For example,at 3 volts and 65° C., Carbon 1 was found to only have an internal cellpressure of about 4.5 bar after 250 hours of operation in comparison toCarbon 3 which was found to have an internal pressure of over 7 bar forthe same amount of operation. This shows that carbon electrodes with alowered surface functional group concentration were found to provide fora device that could perform for longer while reducing the internalpressure within the ultracapacitor package.

Modification of a Carbon Surface Functional Group to Reduce anElectrochemical Potential of the Electrode

One embodiment of the invention includes an ultracapacitor configured tooperate at an operating voltage of 3 volts or greater where theultracapacitor includes one or more electrodes comprising carbonmaterial treated with a reactant gas to enable the one or moreelectrodes to maintain a reduced electrochemical potential duringoperation of the ultracapacitor. For example, one or more surfacefunctional groups of an electrode carbon material can be modified tochange an electrochemical potential of the electrode. In someembodiments, carbon material of an electrode may be treated with areactant gas comprising fluorine, and/or nitrogen to modify a surfacecharacteristic of the carbon. The electrode carbon material treated witha reactant gas comprising fluorine and/or carbon may enable theelectrode to maintain or substantially maintain, during operation of theultracapacitor, an electrochemical potential lower than a voltage atwhich one or more secondary parasitic reactions can be generated,facilitating, for example, improved performance of the electrode atincreased operating voltages. For example, a positive electrode of anultracapacitor comprising a carbon material treated with a reactant gascomprising fluorine and/or nitrogen may be able to maintain orsubstantially maintain, during operation of the ultracapacitor, anelectrochemical potential lower than a voltage at which one or moresecondary parasitic reactions can be generated, facilitating, forexample, an improved performance of the ultracapacitor at increasedoperating voltages.

In some embodiments, electrode carbon material may be treated with areactant gas to add one or more favorable functional groups on one ormore surfaces of the carbon material. For example, one or more surfacesof activated carbon material of an electrode may be modified to improvea wettability, conductivity and/or a resistance of the electrode.

Carbon Surface Coating

In one embodiment of the invention, an ultracapacitor configured toprovide desired operation at an operating voltage of 3 volts or greaterincludes one or more electrodes comprising carbon material that istreated to provide a protective coating one or more surfaces of thecarbon material. In some embodiments, one or more electrodes of theultracapacitor include treated carbon material to provide one or morecoatings on a surface of the carbon material to reduce chemicaldegradation of the carbon surface. A suitable material for protectingthe carbon surface from degradation can include, for example, a materialhaving electrochemical stability at temperatures in which theultracapacitor operates, and/or maintains or substantially maintainscarbon surface area and/or ion mobility within the electrode. Treatedcarbon material may include any carbon material used to make theelectrode, including for example activated carbon, graphite, and/orcarbon black.

For example, the protective coating may comprise a porous ceramicmaterial. In some embodiments, the protective coating comprises ametallic material that can protect the carbon surface and/or provideenhanced conductivity for the treated carbon material (e.g., metallicdecorating of the carbon surface using silver atoms). In someembodiments, the protective coating comprises silicon carbide and/ormetallic oxides (e.g., oxides of tin, oxides of titanium, zinc oxide).

The one or more protective coatings may be applied to the carbonmaterial before the carbon material is incorporated into theultracapacitor electrode, and/or after the carbon material isincorporated into the electrode (e.g., in-situ). For example, an in-situmodification of a carbon surface may comprise use of one or moreelectrolyte additives (e.g., one or more additives as described herein).The one or more additives may form a protective coating on one or moresurfaces of the carbon material, for example enabling a reduction in theformation of byproducts and/or a reduction in carbon surfacecontamination.

Improved 3V Ultracapacitor

One embodiment is an electric double-layer ultracapacitor configured toprovide desired operation at increased operating voltages, for examplean operating voltage of 3 volts (V), can include one or more of thefeatures described herein. The device can operate at 65° C. for morethan 1500 hours, and/or greater than or equal to 500 k cycles, forexample, while maintaining more than 80% of its initial capacitance,and/or less than about 200% of its initial equivalent series resistance.In this embodiment, the electric ultracapacitor includes an electrolytecomprising tetraethyl ammonium tetrafluoroborate and/orspiro-(1,1′)-bipyrrolidinium tetrafluoroborate in acetonitrile at aconcentration of about 0.8 mol/L (M). In addition, the device is placedwithin a housing component having a barrier film on the interior surfaceof the housing that is exposed to the electrolyte. This barrier protectsthe device from reactions between the electrolyte and the metallic innersurface of the capacitor housing.

Within the device is a separator made of a cellulose fiber which alsoreduces the rate of any reaction that may degrade the separator. Inaddition to these features, the device also includes electrodes thathave a carbon layer with an optimized composition, packing density,microporosity, and/or macroporosity and so that the carbon electrodesare configured to provide the most advantageous properties when actingat 3.0 volts.

In addition, the thickness of the positive electrode is made thickerthan the thickness of the negative electrode anode to provide desiredoperation at increased operating voltages.

Although this invention has been disclosed in the context of certainembodiments and examples, it will be understood by those skilled in theart that the invention extends beyond the specifically disclosedembodiments to other alternative embodiments and/or uses of theinvention and obvious modifications and equivalents thereof. Inaddition, while several variations of the embodiments of the inventionhave been shown and described in detail, other modifications, which arewithin the scope of this invention, will be readily apparent to those ofskill in the art based upon this disclosure. It is also contemplatedthat various combinations or sub-combinations of the specific featuresand aspects of the embodiments may be made and still fall within thescope of the invention. It should be understood that various featuresand aspects of the disclosed embodiments can be combined with, orsubstituted for, one another in order to form varying modes of theembodiments of the disclosed invention. Thus, it is intended that thescope of the invention herein disclosed should not be limited by theparticular embodiments described above.

The headings provided herein, if any, are for convenience only and donot necessarily affect the scope or meaning of the devices and methodsdisclosed herein.

1.-10. (canceled)
 11. A method of making an ultracapacitor configured tooperate between about 2.8 volts to about 3 volts, comprising: providinga first current collector electrically coupled to a positive electrodeand a second current collector electrically coupled to a negativeelectrode, and separated by a separator into a housing; and adding anelectrolyte to the housing, wherein the electrolyte comprises aquaternary ammonium salt in ionic contact with the positive electrodeand the negative electrode, wherein the electrolyte comprises less than1M salt.
 12. The method of claim 1, wherein the quaternary ammonium saltcomprises a cation selected from the group consisting ofspiro-(1,1′)-bipyrrolidinium, triethylmethyl ammonium and tetraethylammonium.
 13. The method of claim 1, wherein the quaternary ammoniumsalt comprises an anion selected from the group consisting oftetrafluoroborate and iodide.
 14. The method of claim 1, wherein theelectrolyte further comprises acetonitrile.
 15. The method of claim 1,wherein the salt concentration of the electrolyte is about 0.8M.
 16. Themethod of claim 1, wherein the capacitor is configured to operate at 65°C. with a cycle life of greater than 500 k cycles.
 17. The method ofclaim 1, wherein the capacitor is configured to operate at 3 volts forgreater at 65° C. and maintaining greater than 80% of its initialcapacitance for over 1500 hours.
 18. The method of claim 1, wherein thecapacitor is configured to operate at 3 volts for greater at 65° C. andmaintaining less than 200% of its initial equivalent series resistancefor over 1500 hours.
 19. The method of claim 1, wherein the positiveelectrode and negative electrode each independently comprise a dryelectrode film comprising carbon and binder.